The sex of a human fetus and certain fetal chromosomal abnormalities are conventionally detected or confirmed by directly examining the chromosomes in fetal cells by cytogenetic analysis or by testing for specific sequences of DNA within the chromosomes using nucleic acid analysis. These tests require the collection and culturing of living cells obtained through an outpatient surgical procedure involving some risk to the mother or fetus. Cells, which have been shed from the fetus, may be obtained by amniocentesis. Amniocentesis involves inserting a needle through the abdominal wall into the uterus and withdrawing a small amount of amniotic fluid. An alternative procedure involves sampling the tissue of chorionic villi from the surface of the placenta by inserting a catheter through the cervix or abdomen. However, spontaneous miscarriage or other serious complications may occur in about 0.5% of amniocentesis procedures and about 1% of chorionic villus procedures. Fetal cells collected by amniocentesis or chorionic villus sampling are grown in culture for several days and then examined for abnormalities.
Various kinds of fetal cells have been characterized. Fetal cells include, but are not limited to, fetal erythrocytes, lymphocytes or trophoblasts. Trophoblasts include cytotrophoblast or syncytiotrophoblast cells and cells which may be sampled from embryos produced by in vitro fertilization techniques. As used herein, the term "erythrocytes" includes erythroblasts, normoblasts and reticulocytes, as well as erythrocytes, unless the contrary is clear from the context.
It is known that a small number of fetal cells circulate in the mother's blood. About one in 4,000 to 7,000 fetal erythrocytes in the maternal blood circulation is a fetal nucleated red blood cell. Methods for detecting certain of the fetal cells and/or separating them from the mother's blood have been reported. See, e.g., S.C. Yeoh et al., Prenatal Diagnosis 11:117-123 (1991); U. W. Mueller et al., Lancet 336:197-200 (1990) (isolation of fetal trophoblasts by murine monoclonal antibodies); J. O. Price et al., Am. J. Obstet. Gynecol. 165:1731-37 (1991) (fetal nucleated erythrocytes were flow sorted on the basis of four parameters: cell size, cell granularity, transferrin receptor, and glycophorin-A cell surface molecule); PCT Publication No. WO 91/07660 to Childrens Medical Center Corp. (a method for isolating fetal nucleated erythrocytes by means of an antigen present on the cell surface of the fetal erythrocytes); PCT Publication No. WO 91/16452 of Cellpro Incorporated; and U.S. Pat. No. 5,153,117 (a method for selectively recovering fetal cells from a maternal blood sample where cells of the sample are combined with a first and second antibody labeled with different fluorochromes).
Nucleic acid hybridization techniques are based on the ability of single-stranded DNA or RNA to pair, i.e. hybridize, with a complementary nucleic acid strand. This hybridization reaction allows the development of specific probes, or populations of probes, that can identify the presence of specific genes (DNA) or polynucleotide sequences of the transcription of those genes (RNA).
By the use of specific nucleic acid (RNA or DNA) probes, genetic markers for the gender or other genetic characteristic of the fetus and for infection and other disease states may be detected. Certain genetic diseases are characterized by the presence of genes absent in normal tissue. Other disease conditions are characterized by the expression of RNAs or RNA translation products (i.e. peptides or proteins) which are not expressed in normal cells. Some disease states are characterized by the absence of certain genes or portions of genes, or the absence or alteration of expression of gene products or proteins. Moreover, it is often desired to characterize the gender of animal fetuses, such as bovine fetuses, as well as human.
For background on nucleic acid genetic testing, see e.g., P. G. McDonough, Sem. Perinatol. 9:250-256 (1985), and W. G. Butler, et al, Fertility & Sterility 51:375-386.
Solution hybridization methods require the destruction of the cell and the isolation of the nucleic acids from the cell before carrying out the hybridization reaction. These methods sacrifice cellular integrity, spatial resolution and sensitivity of detection. Where relatively few cells are available for isolation, as with fetal cells circulating in maternal blood, solution hybridization is not feasible.
Amplification of nucleic acids, such as by the polymerase chain reaction, is a known technique, but with certain known drawbacks preventing optimal speed and efficiency. For example, such techniques may cause lysis of cells, may produce false positives due to sensitivity of the technique, and may lead to loss of specificity where high levels of amplification are required to detect a target that is present in low copy number. Moreover, hybridization of the amplified target is required in any event, so that multiple time-consuming steps are performed when amplification is used.
In situ hybridization provides a technique for the determination and quantitation of nucleic acids (DNA and RNA) in tissues at the single-cell level. Such hybridization techniques can detect the presence or absence of specific genes in cells and may also be utilized to detect the expression of gene products at the single-cell level.
In situ hybridization procedures are disclosed in U.S. Pat. No. 5,225,326 and copending U.S. patent application Ser. No. 07/668,751, now abandoned. The disclosure of each patent, patent application and journal publication identified in this patent application is incorporated by reference.
Despite the aforementioned knowledge, the prior art remains deficient. A truly rapid, sensitive, efficient and practical method of determining fetal gender and of detecting fetal genetic abnormalities on a routine basis without invading the mother's womb is lacking. Thus, the present invention fulfills a long-felt need and desire in this field.